B and T cell hybrid culture lines: Fusion of myeloma cells with normal antibody producing cells, in order to obtain clonal B cell hybrid culture lines which secrete large quantities of monoclonal antibodies (Mabs) reactive with H-2, Ia and other lymphocyte surface determinants; fusion of T lymphomas with normal T cells in order to obtain clonal T cell hybrid culture lines which continue to express characteristic T cell determinants and/or functional properties; use of these lines and their products for genetic, chemical and functional studies of various lymphocyte phenotypic markers. Lymphocyte surface determinants and subpopulations: Identify and characterize lymphocyte cell surface determinants, especially those which mark functional lymphocyte subpopulations or progressive stages of differentiation of individual subpopulations; functionally characterize lymphocyte subpopulations carrying different surface markers; genetically map loci controlling lympyocyte surface determinants. Selective expression of I-region loci: Understand the mechanism and functional significance of the selective expression of loci in the I-region of the major mouse histocompatibility complex (H-2) which control determinants on functional T cell subpopulations.